Question

I want to execute background processes concurrently from a lua script

like :

a = io.popen("deploy.exp" .. ip1):read("*a")
b = io.popen("deploy.exp" .. ip2):read("*a")

where a,b are continually running processes. When i do this as above, b will only run when a is finished. And the deploy.exp script is an expect script which used to ssh few servers, and execute some commands. Then I need to fetch some text from a and b. Any idea on this? I tryed with the ExtensionProposal API. When I tried that I get one error messages that say: “* glibc detected free(): invalid next size (fast): 0x08aa2300 ** abort”.

The part code is

for k,v in pairs(single) do
command =  k .. " 1 " ..  table.concat(v, " ")
local out = io.pipe()
local pro = assert(os.spawn("./spaw.exp " .. command,{
      stdout = out,  
}))
if not proc then error("Failed to aprogrinate! "..tostring(err)) end
print(string.rep("#", 50))
local exitcode = proc:wait()
end

Has anybody any experience (or advice / where we should look) with this? or give me a sample? Thanks

BTW: I tried the luaposix, but I can’t find any sample by posix.fork(). Does anyone could share one? TKS

Answer 1

posix.fork() is part of the luaposix library, which can be installed via luarocks. It works in much the same way as fork(3); it creates an copy of the parent process, and both of them will execute everything after the call to fork(). The return value of fork() is 0 in the child process, otherwise it’s the PID of the child that was just spawned. Here’s a contrived example:

local posix = require "posix"
local pid = posix.fork()

if pid == 0 then 
  -- this is the child process
  print(posix.getpid('pid') .. ": child process")

else 
  -- this is the parent process
  print(posix.getpid('pid') .. ": parent process")

  -- wait for the child process to finish
  posix.wait(pid) 

end

-- both processes get here
print(posix.getpid('pid') .. ": quitting")

This should output something like the following:

$ lua fork.lua 
27219: parent process
27220: child process
27220: quitting
27219: quitting